1. Field of the Invention
This invention relates generally to the field of measurement of field leakage in a flyback transformer; and, more particularly, it relates to a sensing circuit utilizing a Giant Magnetoresistive Ratio (GMR) device to sense field leakage of a flyback transformer. Still more particularly, the invention relates to a sensing circuit for use in controlling the capacitor charging functions of an Implantable Cardioverter Defibrillator (ICD).
2. State of the Prior Art
Various types of circuits utilize flyback transformers to perform a DC/DC conversion that permits a step up from a source voltage to a required amplified voltage. Flyback transformers employ a primary winding, a ferromagnetic core, and one or more secondary windings. Current is driven through the primary winding by a current driving circuit so that electrical energy is transformed in the core to magnetic energy until such time as the core nears saturation. Once the core nears saturation, the current driving circuit opens the primary winding. With the primary winding opened, the primary current loses its original path. Since the energy in the magnetic field of the flyback transformer is proportional to the current squared, and since energy must be conserved in the closed system of the flyback transformer, a secondary current must appear through the secondary winding to conserve the total flyback transformer energy. The secondary current then continues to flow until the magnetic field has collapsed. In the case of a capacitor charging circuit, the secondary current flows into and charges a capacitor connected to the secondary winding, typically through a blocking diode.
Upon saturation, the primary winding appears nearly as a short circuit to the current driving circuit that is providing power to the primary winding, thus subjecting the current driving circuit to severe, damaging current carrying conditions. This also causes excess heat build-up in the flyback transformer which can cause damage to the primary winding and lower the flyback transformer charging efficiency. To avoid this damage, it has been found to be advantageous to have the current driving circuit interrupt current flow in the primary winding just prior to saturation of the core to maximize magnetic energy storage in the core while avoiding the high current carrying conditions.
With the flyback transformer approach, there are many limitations on efficiency. For example, while the use of ferromagnetic core materials result in increased energy densities, the resultant hysteresis losses occurring during switching adds to the power consumption requirements of the current driving circuits. That is, the hysteresis loss decreases the charging circuit efficiency and more energy must be applied to the flyback transformer to make up for the lost energy. This can be especially problematic for battery powered charging systems. The rate of switching can also have an effect on hysteresis losses, as well as the rate of heating that can occur in the flyback transformer. The charging circuit must accommodate these inefficiencies, which can result in excessive capacitor charging times and decreased battery lifetimes for battery powered charging systems.
One type of device that utilizes flyback transformers is the Implantable Cardioverter Defibrillator (ICD). This type of device is implanted in the body of a patient, and utilizes a charged capacitor to administer electrical pulses to the patient's heart. In general, the ICD has a battery coupled to a charging circuit that includes a flyback transformer for charging one or more capacitors interconnected in series to provide a high voltage output. By using the flyback transformer, a battery voltage, for example, on the order of about six volts DC, in a series of interrupted steps can cause the capacitor (or series of capacitors) to be charged to a level of about 750 volts DC, thereby providing a voltage level capable of providing an adequate charge to stimulate the patient's heart for the purpose of defibrillating the heart.
The prior art has recognized the desirability of limiting current in the primary winding of the core of a flyback transformer to as near to saturation as possible to maximize magnetic energy storage in the core while avoiding the problems caused by the high current flow levels in the primary charging circuit during saturation. Existing solutions have included measuring current flow to allow estimation of sufficiency of primary current flow necessary to bring the transformer core to near saturation. Since each transformer structure varies somewhat as a result of manufacturing tolerances, this approach requires having a safety margin to avoid saturation resulting in decreased efficiency. The choice of frequency of operation of the transformer may also impact the safety margin as there is a tradeoff between charging efficiency and transformer size, since high frequency switching allows small transformer sizes but increases hysteresis losses. Further, any heating of the flyback transformer also reduces efficiency, and thus impacts the safety margin. As a result, this approach to improve the efficiency of the flyback transformer by measuring primary winding current flow necessarily has lower efficiency once the safety margin is increased to accommodate the various factors discussed above.
Another prior art approach to improve the efficiency of the flyback transformer is by controlling the duration of the application of power to the primary winding of the flyback transformer. With the timed approach, various factors are considered such as the characteristics of the battery supply, the primary current driving circuit capability, the flyback transformer size and efficiency, and the switching frequency., all in order to estimate the fixed time each primary charging cycle that power should be applied to the primary winding. This approach however has many of the same problems as the above approach of measuring primary winding current. Again, as with the above approach, once the various factors are accommodated to build in a safety margin to the fixed time necessary to charge the primary winding to avoid saturation, the result is a less than optimal energy transfer.
While the above problems in the prior art relate to the optimal interruption of current flow in the primary winding just prior to saturation of the core to maximize energy transfer, yet another problem exists in the prior art regarding the optimal reapplication of power to the primary winding after it has been removed.
In the prior art, various approaches have been used to determine when to optimally reapply power. When the power applied to the primary winding is switched off for example, the voltage across the primary winding decreases as the field collapses. Once the voltage across the primary coil passes through zero, it is possible to again apply power in the primary circuit to start the next cycle of magnetic energy buildup via the primary winding. Thus one approach in the prior art has attempted to detect the zero volt cross-over of the primary winding to provide activating signals to enable the primary charging circuit. This approach however is not optimal due to the high degree of difficulty in measuring the voltage transition point due to the high rate of transition of the voltage across the primary.
Other approaches have attempted to measure the voltage across the secondary winding while the magnetic energy in the collapsing magnetic field is being converted to electrical energy stored in the capacitor. With these approaches, an attempt is made to determine when the secondary current induced in the secondary winding has been discharged into the capacitor so that power may be reapplied to the primary winding. Often times these approaches are not optimal due to the difficult in measuring the secondary voltage to determine the precise time at which the secondary current is equal to zero so that the primary power may be reapplied. Approaches using a fixed time delay or fixed voltage reference are still not optimal for the reasons discussed earlier.
Yet other problems exist in the prior art with regards to ICDs. When the capacitor is discharged in a ICD, the initialization of charging requires special handling. As described above, it is common to use the voltage measured across the primary winding as a determining factor to recognize when power can be reapplied to the primary winding. Unfortunately, this voltage measurement approach is not available during the initial charging cycles of the primary winding. As the capacitor is charged, the stored voltage level across the capacitor is reflected back through the secondary winding to the primary winding. When the sequence of charging cycles first start, the reflected primary voltage swing is too small to accurately detect the zero crossing threshold. To accommodate this condition during the initial charging cycles, a predetermined time delay has been utilized. For the reasons discussed above, this approach is not optimal.
What is desired is an approach to determine when to remove and when to reapply power to the primary winding which is not subject to the inherent inefficiencies of the above approaches, and which is not affected by ambient or operating conditions or manufacturing tolerances.